Relocking devices in current use are normally based on a construction which employs a round plunger or bolt. The bolt is normally held in the inoperative or waiting condition by some form of constraining means attached either directly or indirectly to the locking means or locking points. When the locking means or locking points are attached the constraining means is released and the relocking device is triggered into the locked position and hence dogs or "relocks" the master boltwork into its locked condition making it much more difficult to defeat the security closure.
In a well planned attack against a safe door or like security closure an attempt will be made to nullify the relocking mechanism by such methods as pinning back the relocking bolt or burning the end of it away with oxy-acetylene equipment. In some cases the relocking mechanism is allowed to trigger, and after the attack on the main locking mechanism is complete the relocking bolt is "fished" back by suitable probing tools to allow the door to be opened.
If the size or engagement of the relocking device is nor very substantial, then the device becomes very susceptible to being burnt away by a thermal attack, and indeed such devices can be defeated in a very short time by the use of a thermic lance. To be effective a relocking device ought to be of a design which permits a considerable length of engagement with the main boltwork and which also makes it very difficult to "fish"the relocking bolt back to the inoperative position.